An example of a method for designing a semiconductor device such as SOC is a standard cell method. In this designing method, a plurality of cells (also referred to as logic cells or standard cells, for example) are prepared for individual functions, and a semiconductor device is fabricated using them as its components. In the case of a standard cell method, steps of arranging cells and connecting wirings between the cells are performed with an automatic placer and router.
A variety of semiconductor devices that take advantage of the extremely low off-state current of a transistor whose semiconductor region is formed using an oxide semiconductor (hereinafter, such a transistor may be referred to as an OS transistor) have been proposed.
For example, Patent Documents 1 and 2 each disclose a storage circuit using an OS transistor. Non-Patent Document 1 discloses a processor that is capable of power gating and in which backup circuits using OS transistors are provided in a flip-flop and an SRAM. Patent Document 3 discloses a semiconductor device using a combination of a standard cell including a Si transistor and an OS transistor and a standard cell including a Si transistor.